The present invention relates to a rotating anode for an X-ray tube and, more particularly, to means for avoiding the anarchical creation of cracks in a target supported by the anode.
With X-ray tubes, X-radiation is currently obtained by electronic bombardment of an anode. More specifically, electronic bombardment is focused onto a small area referred to as the focal point of a target enclosed in the anode, which focal point becomes the source of X-radiation.
A small part of the electrical energy which is used to accelerate the electrons (approximately 1%) is transformed into X-rays. The remainder of this energy is dissipated into heat. This heat, mainly evacuated by radiation, can lead to the deterioration of the anode and, more particularly, to the deterioration of the target, such as the melting of the focal point of the target.
Consequently, the target is generally made of a material which not only has a high atomic number to promote the generation of X-rays, but also a refractory material which is a good conductor of heat, e.g. tungsten or molybdenum or alloys thereof, etc.
However, whatever the material of that the target is made of, the instantaneous power levels involved (in the order of 100 kW) create major stresses in the surface layers of such a material.
To decrease the temperature at the focal point, a current solution consists in having the target run under the focal point or the impact of the electron beam. This movement of the target is obtained by a rotation of the anode about an axis of symmetry of the latter, with the anodes being generally in the form of a disk. The movement of the target beneath the focal point created on impact of the electron beam generates, on the target and around the axis of symmetry, a focal crown or ring of several millimeters in width.
A fast rotation of the anode, (several thousands of revolutions per minute) is required to distribute the thermal flow around the focal crown. But the temperature of the focal point remains far higher than the temperature of the remainder of the focal crown, which itself has a temperature well above that of the remainder of the anode disk.
It is observed that at each point of this focal crown a "thermal pulse" is received on each revolution of the anode. With the materials generally used for the emission of X-radiation under the effect of electronic bombardment, i.e. target materials such as tungsten typically, fluctuations due to such pulses may be considered as insignificant beyond a surface layer, the thickness thereof being in the order of 100 microns. Accordingly, it is essentially this surface layer which is subjected to a series of thermal shocks due to the rotation and, consequently, to major mechanical stresses.
Further, at another time scale, there may be a pause which may last for example 0.1 second to 1 second or even more, while the entire focal crown receives considerable thermal flow that only diffuses gradually throughout the entire anode disk.
Consequently, the inventors have thought that the focal crown is subjected to a major compression due to a major expansion of the target material and that, the target material is likely to come out of the elasticity range of the material so that a tensile stress resulting from cooling may generate cracks in the surface of the material of which the target is made.
These cracks tend to increase in number and scale with the operating time, and become detrimental to the correct operation of the X-ray tube: thus for example, in the case of an anode made of a basic body (typically of graphite), coated with a layer of X-ray emitting material or target material (e.g. tungsten), such cracks may extend to the graphite and lead to the lifting-off of the layer of tungsten resulting in the fast destruction of the tube; it is also be noted that such cracks if too numerous, tend to reduce the amount of X-rays emitted by the focal point.